Heterosis is the term used to describe the superior performance of F1 hybrids over parental lines from which the F1 is derived. In crop plants heterosis often manifests in the production of larger plants, tolerance to stress, disease resistance, uniformity and improved yield, which are collectively referred to as hybrid vigour.
Heterosis has been observed and documented in many important crop species and the development of hybrids by plant breeders is established practice. However, hybrids can only be used in crops when effective and economical means of pollination control exist to ensure cross pollination and prevent self-pollination. Pollination control mechanisms include mechanical, chemical and genetic means.
A mechanical means for hybrid plant production can be used if the plant of interest has spatially separate male and female flowers or separate male and female plants. For example, a maize plant has pollen-producing male flowers in an inflorescence at the apex of the plant, and female flowers in the axiles of leaves along the stem. Outcrossing of maize is assured by mechanically detasseling the female parent to prevent selfing. Even though detasseling is currently used in hybrid seed production for plants such as maize, the process is labor-intensive and costly, both in terms of the actual detasseling cost and yield loss as a result of detasseling the female parent. Further, most major crop plants have both functional male and female organs within the same flower, and therefore emasculation is not a simple procedure. While it is possible to remove by hand the pollen forming organs before pollen is shed, this form of hybrid production is extremely labor intensive and expensive.
Chemical means of producing hybrid plants involves the use of chemicals that kill or block viable pollen formation. These chemicals, termed gametocides, are used to impart a transitory male-sterility. Commercial production of hybrid plants by use of gametocides is limited by the expense and availability of the chemicals and the reliability and length of action of the applications. A serious limitation of gametocides is that they have phytotoxic effects, the severity of which are dependent on genotype. Other limitations include that these chemicals may not be effective for crops with an extended flowering period because new flowers produced may not be affected. Consequently, repeated application of chemicals is required.
Many current commercial hybrid plant production systems for field crops rely on a genetic means of pollination control. In such systems, plants that are used as females either fail to make pollen, fail to shed pollen, or produce pollen that is biochemically unable to effect self-fertilization. Plants that are unable to self-fertilize are said to be “self-incompatible” (SI). Difficulties associated with the use of a self-incompatibility system include availability and propagation of the self-incompatible female line, and stability of the self-compatibility. In some instances, self-incompatibility may be overcome chemically, or immature buds can be pollinated by hand before the biochemical mechanism that blocks pollen is activated. Self-incompatible systems that can be deactivated are often very vulnerable to stressful climatic conditions that break or reduce the effectiveness of the biochemical block to self-pollination.
Genetic systems involving cytoplasmic or nuclear genes may be used to generate male sterility. Cytoplasmic male sterility (CMS) is at present the most widely used mechanism of pollen control in crops. However, CMS has a number of disadvantages including increased disease susceptibility, breakdown of sterility under certain conditions, undesirable characters linked to restorer genes (genes that can suppress the male-sterile effect of the cytoplasm and are incorporated into the male parent to restore pollen fertility in the F1 hybrid), unreliable restoration, etc. Crops in which such problems occur are, for example, maize, oilseed rape and wheat.
Consequently, plant breeders and seed producers require a versatile and durable male sterility system. The genetic engineering approach has a number of advantages over natural systems. The disruption of the genotype of new male-sterile plants normally associated with the introduction of sterility by sexual means is completely avoided and sterility is not linked to particular cytoplasms as in cytoplasmic male sterility.
Nuclear-encoded male sterility (NMS) is caused by mutations in the nuclear genome. The advent of plant genetic engineering technology has now made it feasible to develop strategies to permit the use of NMS genes.
Plant breeders and seed producers require a versatile and durable male sterility system. The problems that must be overcome include the isolation of genes that induce male sterility, the production of 100% male-sterile progeny, the achievement of complete female fertility and subsequent restoration of pollen fertility in the F1 hybrid.
An aim of the present invention is to determine gene(s) involved in pollen formation and develop methods of preventing pollen formation by disrupting the expression or activity of the gene(s) or their gene product(s). A further aim of the present invention is to develop a method for the production of transgenic male sterile plants and/or seeds and harvest plants and/or seeds so produced.